In this article,
we demonstrate that a first-order spin penalty
scheme can be efficiently applied to the Slater determinant based
Full-CI Quantum Monte Carlo (FCIQMC) algorithm, as a practical route
toward spin purification. Two crucial applications are presented to
demonstrate the validity and robustness of this scheme: the
1
Δ
g
←
3
Σ
g
vertical excitation in O
2
and
key spin gaps in a [Mn
3
(IV)
O
4
] cluster.
In the absence of a robust spin adaptation/purification technique,
both applications would be unattainable by Slater determinant based
ground state methods, with any starting wave function collapsing into
the higher-spin ground state during the optimization. This strategy
can be coupled to other algorithms that use the Slater determinant
based FCIQMC algorithm as configuration interaction eigensolver, including
the Stochastic Generalized Active Space, the similarity-transformed
FCIQMC, the tailored-CC, and second-order perturbation theory approaches.
Moreover, in contrast to the GUGA-FCIQMC technique, this strategy
features both spin projection and total spin adaptation, making it
appealing when solving anisotropic Hamiltonians. It also provides
spin-resolved reduced density matrices, important for the investigation
of spin-dependent properties in polynuclear transition metal clusters,
such as the hyperfine-coupling constants.